This invention relates generally to the art of laser beam interferometers, and more particularly to Fizeau heterodyne interferometers.
As is known in the art, heterodyne interferometers in general have a wide variety of applications. One such application is in laser doppler velocimeters (LDVs) for radar systems. Heterodyne interferometers of the Mach-Zehnder type have been utilized successfully in laser doppler velocimeters. As is known, the local oscillator (i.e. reference) beam travels along a separate path from the beam transmitted to the target and the target-reflected return beam in a Mach-Zehnder heterodyne interferometer. The target return beam and the reference beam are combined in a recombining beamsplitter and directed onto an optical detector. Such interferometer is thus a relatively large device and hence may be unsuitable for applications wherein size is of primary importance. One possible substitution is the Fizeau heterodyne interferometer, a device wherein the target-reflected return beam, and the local oscillator (i.e. reference) beam travel along a common optical path to an optical detector, thus achieving a compact structure.
In a conventional Fizeau heterodyne interferometer the local oscillator beam power is fixed at approximately the product of the power of the laser beam and the reflectivity of a beamsplitter which reflects the laser beam to produce the local oscillator beam. The operation of the optical detector is a function of the power of the local oscillator beam which illuminates the detector. If the local oscillator beam power is too high the optical detector will not function optimally, resulting in reduced interferometer signal-to-noise ratio and a corresponding marked decrease in LDV performance. Additionally, the optimum operating power may vary from detector to detector. One way of reducing the local oscillator beam power is to utilize a very low reflectivity beamsplitter to reflect the laser beam and produce the local oscillator beam. However, repeatable production of such very low reflectivity beamsplitters is unreliable since the reflectivity thereof typically varies from beamsplitter to beamsplitter. Another method of decreasing local oscillator beam power is to place an optical attenuator in the path of the local oscillator beam. However, since the local oscillator beam travels along a common path with the target reflected return beam in a Fizeau interferometer, the optical attenuator would also weaken the target-reflected return beam, thus reducing the sensitivity of the interferometer.